Typical Vertical Vacuum Furnace

A number of brazing shops today combine brazing and heat-treatment in their vacuum furnaces to join components together and then obtain certain desired base-metal properties in those brazed components via rapid cooling (quenching) immediately after brazing is done, and before the components are removed from the furnace. Thus, the same vacuum-furnace brazing cycle combines brazing and heat-treatment, yielding clean brazed components with special base-metal properties to meet unique end-use service conditions.

Vacuum furnaces today offer a number of options regarding the introduction and use of circulating gases in the furnace hot-zone during brazing processes. Gases typically introduced into the vacuum furnace are either argon or nitrogen, but a number of shops have found success with hydrogen and helium gases as well. Often gases are combined rather than just using one gas. Hydrogen/nitrogen and helium/nitrogen are typical combinations used in some brazing shops.

The heat-treat literature offers a lot of valuable information about the methods and effects of heat-treatment on metals based on the gas-atmospheres used, as well as on the cooling/quench rates employed to achieve desired end-use properties.

What I find missing in any books or articles about combining brazing with heat-treatment is any guidance as to when the quenching should actually begin during the furnace cycle. Let me offer some suggestions in this article, based on what I’ve seen many times in induction-brazing and torch-brazing, where quenching has had bad effects on the brazed joint when used incorrectly.

I always recommend to people that (in the absence of other knowledge or experience) they should braze at about 100F (50C) above the published liquidus-temp of the particular brazing filler metal (BFM) being used. This amount of “superheat” should allow the BFM to flow nicely into and through the joint being brazed, and effectively diffuse into (bond with) the base metal surfaces being joined.

Then, following the brazing process, the molten BFM should be allowed to cool in place until it is completely solidified. It is VERY important that this molten filler metal NOT be disturbed during cooling, since any such disturbances can introduce cracks and voids into the cooling BFM, resulting in poor bonding and weakened braze-joints that can leak (if the joint is supposed to be pressure-tight or leak-tight) or fail in service when subjected to any type of thermal or mechanical stresses.

I observed this recently when a brazing shop was using induction brazing to join cylindrical fittings together, and, for the purposes of speeding up production, blasted the induction-brazed joint with a rapid stream of argon gas the instant the induction-braze was completed (the BFM in the joint was still clearly molten). I was brought to their shop to help them determine why that particular assembly had a very high incidence of leakers at that brazed joint. As soon as I saw what they were doing with the quench-gas, I realized where their problem lay. I advised them to wait just a few seconds until the operator could see that the BFM had fully solidified in the joint before applying the quench-gas to the component. The leak-problem disappeared.

My recommendations when combining furnace-brazing and HT:

1. Brazing filler metals (BFMs) must be allowed to cool undisturbed from its molten state until its temperature is below the solidus-temperature of that BFM.

Thus, BFMs should never be quenched while they are still molten during any brazing operation (be it torch, induction or furnace brazing). BFMs must simply never be quenched while they are still molten, since such rapid cooling of any molten BFM can disturb the solidification process, causing the BFM to move and shift while trying to solidify, resulting in excessive voiding, cracking, etc. in the joint being brazed.

2. Experiment with your furnace cycles to determine best timing to begin your rapid-cooling/quench to insure non-disturbed BFM solidification.

You may find that, because of the normal delays in your furnace control instrumentation, the molten BFM may have fully solidified before the cooling gases actually hit the parts being brazed. If so, fine. But, if your quenching capabilities are such that that cooling gas may hit those parts while the BFM is still molten, then you may need to adjust the cooling cycle timing a little before the gas is introduced.

KEY TO SUCCESS: Do not disturb the joint in any way while molten BFM is liquid and starting to cool down. Allow it to solidify completely before quenching it or moving it. Careful adherence to this principle will prevent a lot of potential problems in brazing.

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